104
and Capone 1989). However, additions of specific compounds cannot replicate the
complex mixtures present in seawater, which can complicate interpretations from
these experiments. The roles of DOM in N 2 fixation continue to be one of the major
open questions and challenges in understanding controls on N 2 fixation.
Recent evidence indicates that certain marine diazotrophs can respond to and
effectively metabolize some petroleum hydrocarbons (Karthikeyan et al. 2020; Shin
et al. 2019)(See Chap. 9).
6.6 Salinity
The requirement of bacteria, and particularly marine bacteria, for inorganic salts has
been a longstanding area of marine microbiological research (MacLeod 1965). A
number of studies have examined at the broad scale the possible role of salinity
(including the major ionic species such as sulfate) in differences in N 2 fixation and
the relative density of heterocyst-forming cyanobacteria between lake and marine
environments (Howarth and Marino 1988; Howarth et al. 1988; Marino and Howarth
2016). Far fewer studies have experimentally considered the effects of salinity on N 2
fixation. An early study examined the effects of salinity on a marine heterotrophic
bacterium related to Azotobacter (Dicker and Smith 1981) showing an optimum in
nitrogenase activity and respiration at mid-salinities for this bacterium. The studies
of Howarth, Marino and colleagues mentioned above (Marino and Howarth 2016)
included studying the effects of salinity in experimental mesocosm studies in estuaries. Thus, salinity does play a role in the physiology of individual microorganisms. However, at ecosystem scales, salinity (such as in estuaries, Chap. 7) is
correlated with multiple factors and has complex effects on N 2 fixation.
6.7 pH
pH has generally not been a major factor considered as affecting non-calcareous
microorganisms in the oceans because seawater is well-buffered by bicarbonate
equilibria at a pH of approximately 8 (Sarmiento and Gruber 2006) and because
microorganisms are able to adapt to natural gradients in pH that occur in the presentday ocean (Joint et al. 2011). Nonetheless, low O 2 waters can have substantially
lower pH (Gobler and Baumann 2016).
More importantly, pH is projected to decrease as a function of increasing dissolved CO 2 from the atmosphere (Jiang et al. 2019) (see Chap. 9). This increase in
total CO 2 shifts inorganic carbon speciation such that the concentration of carbonate
ions decreases, affecting organisms and microorganisms with calcareous plates or
structures, such as corals and foraminifera (Hutchins and Fu 2017). Additionally,
increases in dissolved CO 2 in the ocean are expected to benefit some photosynthetic
6 Factors Controlling N 2 Fixation
and Capone 1989). However, additions of specific compounds cannot replicate the
complex mixtures present in seawater, which can complicate interpretations from
these experiments. The roles of DOM in N 2 fixation continue to be one of the major
open questions and challenges in understanding controls on N 2 fixation.
Recent evidence indicates that certain marine diazotrophs can respond to and
effectively metabolize some petroleum hydrocarbons (Karthikeyan et al. 2020; Shin
et al. 2019)(See Chap. 9).
6.6 Salinity
The requirement of bacteria, and particularly marine bacteria, for inorganic salts has
been a longstanding area of marine microbiological research (MacLeod 1965). A
number of studies have examined at the broad scale the possible role of salinity
(including the major ionic species such as sulfate) in differences in N 2 fixation and
the relative density of heterocyst-forming cyanobacteria between lake and marine
environments (Howarth and Marino 1988; Howarth et al. 1988; Marino and Howarth
2016). Far fewer studies have experimentally considered the effects of salinity on N 2
fixation. An early study examined the effects of salinity on a marine heterotrophic
bacterium related to Azotobacter (Dicker and Smith 1981) showing an optimum in
nitrogenase activity and respiration at mid-salinities for this bacterium. The studies
of Howarth, Marino and colleagues mentioned above (Marino and Howarth 2016)
included studying the effects of salinity in experimental mesocosm studies in estuaries. Thus, salinity does play a role in the physiology of individual microorganisms. However, at ecosystem scales, salinity (such as in estuaries, Chap. 7) is
correlated with multiple factors and has complex effects on N 2 fixation.
6.7 pH
pH has generally not been a major factor considered as affecting non-calcareous
microorganisms in the oceans because seawater is well-buffered by bicarbonate
equilibria at a pH of approximately 8 (Sarmiento and Gruber 2006) and because
microorganisms are able to adapt to natural gradients in pH that occur in the presentday ocean (Joint et al. 2011). Nonetheless, low O 2 waters can have substantially
lower pH (Gobler and Baumann 2016).
More importantly, pH is projected to decrease as a function of increasing dissolved CO 2 from the atmosphere (Jiang et al. 2019) (see Chap. 9). This increase in
total CO 2 shifts inorganic carbon speciation such that the concentration of carbonate
ions decreases, affecting organisms and microorganisms with calcareous plates or
structures, such as corals and foraminifera (Hutchins and Fu 2017). Additionally,
increases in dissolved CO 2 in the ocean are expected to benefit some photosynthetic
6 Factors Controlling N 2 Fixation